The present invention relates generally to sensor technology. More particularly, the present invention relates to remote devices used to record the conditions of many items and that allow wireless interrogation to determine their identity and state.
In many sensing applications, it is desirable to determine whether an event has occurred. This event may include an over-limit such as surpassing a property threshold (i.e. a detrimental concentration of a bacteria in food) or may include a time integrated exposure, a material phase change, etc. In some sensing applications, it may be desirable to determine whether multiple events have occurred. The multiple events may include surpassing different thresholds for a property or combinations of multiple events for different properties. In many applications, monitoring an event as the event occurs may not be possible or practical, and thus it may be desirable to determine whether the event has occurred afterwards. In addition, many applications require that numerous items be separately analyzed to determine whether the event has occurred for any of the items. Further, the remote nature of many sensor applications, such as a sensor placed within a sealed container, may demand a wireless form of communication.
The majority of current wireless monitoring systems provide real time response which may not be suitable or necessary in applications where a well defined event is being detected. For the case of detecting an infrequent event, continual real time information feedback for a large number of items may be inefficient. In addition, the complexity and size of real time sensors may make application in many environments unpractical.
Many sensing applications provide significant challenges due to inaccessible, environmentally prohibitive, or functionally disadvantageous conditions. Current device designs often cannot meet this need. Active sensors have been combined with various forms of wireless data communication, but these devices are generally large and require a battery or other subsequent power source. Further, the battery power source and/or attendant wiring may have a limited range of operation, thereby making the system fragile.
One particular example of the need for multiple event-recording devices for a large number of items is in developing faster methods for inspecting and maintaining the structural and functional integrity of large systems. Such systems include highways, bridges, buildings, aircraft, food or waste products, and reusable launch vehicles (RLVs), such as the Space Shuttle. One type of primary failure mode that can affect the Space Shuttle thermal protection system (TPS) tile performance is thermal breach. Thermal breach may be caused by hot gas penetration and over-temperature conditions at the TPS bond line during earth reentry, and results in the loss of gap fillers and/or the dimensional instability of the TPS. Thermal breach is difficult to detect since thermal penetration may damage the interior surface and/or the TPS bond line without clearly showing external indication of damage on the tile""s top surface.
Current shuttle inspection techniques involve visual and manual inspection of each of the gaps between all of the nearly 22,000 tiles using a hand held filler gauge to measure the thickness and depth of spaces between the tiles. The inspection may further include looking for other effects of thermal stress such as surface damage, discoloration, silicon deposits, or texture changes of the TPS coating. Thermal protection tiles are bonded to a vehicle using an organic adhesive. If the organic adhesive (normally a shiny red) appears dull or black, a closer inspection is required to determine the extent of charring. Presently, Space Shuttle recertification for reflight requires tens of thousands of person hours to manually inspect each of the 22,000 shuttle tiles. The substantial cost of TPS inspection ranks second in operations costs only behind the propulsion system.
Not only is the current approach very slow and expensive, but human inspection is inherently error-prone. Repetitive inspection of the thousands of tiles leads to inspector fatigue and greater potential for error. The scaffolding required to inspect the vehicle is additionally costly and time consuming to set up. For the next generation of reusable launch vehicles (RLVs), it is desired to reduce turnaround time to 24 hours. As current detection methods are prohibitively time consuming and expensive, an automated means of post reentry inspection of the TPS is desirable.
One proposed approach to maintaining RLV systems involves the use of discreet active sensors which rely on a power source directly connected to the sensor. Examples of active sensors which have been used to discretely monitor RLV systems such as propulsion and guidance include strain gauges, thermocouples, and fiberoptic sensors. However, the size and complexity of the active sensors do not allow for monitoring of the TPS since the abundant number of tiles would necessitate a prohibitively excessive amount of weight and wiring.
In view of the foregoing, there are desired improved structures and techniques for wireless sensing and recording for multiple objects.
The present invention provides a device that can be interrogated to determine its identity and its state. The state indicates whether one or more particular physical or chemical events have taken place. In effect, the device records one or more physical or chemical events or states. The device states used to record the one or more events may be recycled. The identity of the device allows it to be distinguished from a number of similar devices. Thus this invention finds particular usefulness in the context of an array of devices that can be probed by a wireless interrogation unit. The device tells the interrogator who it is and what state it is in. The devices and interrogation unit may use a logical analysis and reset to determine if an event has occurred. The interrogator can thus easily identify particular items in an array that have reached one or more particular conditions.
Other devices may be used to sense, record and report multiple physical or chemical events or states. The multiple physical or chemical events or states may be different physical or chemical events or states or may be the same physical or chemical state or event occurring numerous separate times. The multiple events or states may occur separately within two interrogations and/or multiple times over numerous interrogations.
In one embodiment for recording multiple events, the recording device assumes multiple distinct states. Each distinct state may be associated with a particular physical or chemical state or event and can be held until another event occurs. Each distinct device state may also be reported by the transponder in response to wireless interrogation. In another embodiment, the multiple device states may be recycled to permit continuous of the device.
In one specific example, the sensor is a temperature sensor and the physical or chemical events or states are exceeding one or more (high or low) threshold temperatures. A suitable device for this purpose may include a circuit as the recording mechanism and fuses in the circuit as the sensor. When a first threshold temperature is exceeded, a fuse opens at least one path through the circuit, thus changing the state of the recording mechanism. In one embodiment, opening the path changes the resonance frequency of the circuit. When a second threshold temperature is exceeded, a second fuse opens another path through the circuit, thus changing the state of the recording mechanism in a different manner than for the first threshold temperature. One way this can be detected is by probing the device with a swept- or stepped-frequency interrogation signal and detecting the peak frequency of the signal sent from the transponder. Similarly, when temperature decreases below a low-temperature threshold, a liquefied material freezes, thereby changing a circuit path from open to closed (or vice versa) as above.
When numerous devices are implemented (in an array for example), the identification and state of each device may be stored in a database or otherwise recorded. Upon subsequent interrogation of a device, the prior state of the device stored in the database may be used to interpret the current state of the device due to one or more events. The database also allows a history for each device to be maintained.
To keep the device small and simple, it is preferably passive; that is, it does not include its own power source. Thus, the transponder component is preferably passive. In the example described, the radio frequency interrogation signal (or a separate energizing signal) may provide the transponder power. The sensor component and/or the recording mechanism are also preferably passive. In some cases, the physical or chemical events or states themselves provide the power for the recording mechanism to record the one or more events. For example, exceeding a threshold temperature melts a fuse in the above example.
In one embodiment for recording a single event, the recording mechanism assumes a first state when the one or more physical or chemical states or events have not been recorded and assumes a second, third, or further state when the one or more events or states have been recorded. Any state can be reported by the transponder in response to a wireless probe. Preferably, the recording mechanism can be reset after interrogationxe2x80x94either physically in the device or logicallyxe2x80x94but cannot spontaneously return from a state used to record an event to the first state when the physical or chemical state or event ceases or the change in the physical or chemical state reverses. Thus, when the temperature drops back below a threshold temperature (the physical event ceases), the recording mechanism will retain information that the device once exceeded the threshold.
The recording mechanism can take many different forms. It may be a mechanical structure, such as a latching structure, a microelectomechanical device, an integrated circuit memory device, an electrical circuit, an optical circuit, and the like. Likewise, the signal transmitted by the transponder may be provided on many different carriers. For example, the signal may be provided on an acoustic wave, a radio frequency wave, an electrical field, a magnetic field, a microwave frequency wave, a light wave, and the like. If the carrier is a wave of appropriate frequency, the transponder may include a modulator and an antenna.
To allow concurrent detection of multiple events or to allow more precise detection of a parameter value (e.g., temperature), a device of this invention may be designed with a plurality of recording mechanisms or sensors, each configured to record or detect a different physical or chemical state or event and/or the same event for a similar object. The different physical or chemical events or states may be associated with different physical properties (parameters) such as temperature and strain. Alternatively, the different physical or chemical events or states may represent different thresholds of a single physical property. For example, the different thresholds of the single property may be two substantially different temperature thresholds. In this way, a maximum or minimum temperature may be precisely bracketed as determined by which of the sensors or recording mechanisms changed states.
In some embodiments, a single structure serves multiple purposes. For example, a resonant electrical circuit may serve as an antenna, a modulator, and a recording mechanism.
One aspect of the invention provides a system for reporting a physical or chemical state or event. The system comprises a device having at least two device states and one of the two device states corresponding to the physical or chemical state or event. The device comprises a sensor for detecting the physical or chemical state or event. The device also comprises a recording mechanism coupled to the sensor and recording the device state such that the device state changes upon occurrence of the physical or chemical event or a change in the physical or chemical state. The device further comprises a transponder configured to transmit a signal indicating the device state when triggered by a wireless interrogation signal. The system also comprises an interrogator for externally probing the state of the device to determine whether the device state has changed. The interrogator is designed or configured to (i) read the device state by providing the wireless interrogation signal (and power) to the transponder, (ii) compare the device state that it has read against a stored device state (the state was stored either in its memory or in an associated computer database) to determine whether the device state has changed and thereby indicate the physical or chemical event or the change in the physical or chemical state, and (iii) update the stored device state with the device state that it has just read.
Another aspect of the invention provides a method for reporting a physical or chemical state or event by using a device that changes a device state when exposed to the physical or chemical state or event. The method comprises exposing the device to a first environment. The method also comprises probing the device with an interrogator to determine its current device state and thereby determine whether the first environment provided the physical or chemical state or event. The method further comprises saving the current device state as a saved device state. The method additionally comprises, without physically resetting the device state, exposing the device to a second environment which potentially could provide the physical or chemical state or event to cause the device state to change. The method also comprises probing the device with the interrogator to determine if its current device state is different from the saved device state and thereby determine whether the second environment provided the physical or chemical state or event.
Still another aspect of the invention relates to a device for reporting a physical or chemical state or event. The device comprises a sensor for detecting the physical or chemical state or event without using a power source. The device also comprises a recording mechanism coupled to the sensor for recording that the physical or chemical state or event has occurred, wherein the recording mechanism allows recording of multiple physical or chemical events or state changes, each associated with a distinct device state. The device additionally comprises a tag that contains identification information that can distinguish said device from a plurality of similar devices. The device further comprises a transponder, coupled to the recording mechanism and the tag, configured to transmit a signal indicating (i) the device state and thereby indicating the physical or chemical state or event and (ii) the identification information when triggered by a wireless interrogation signal.